Non-nitroglycerine explosives based on Ammonium nitrate such as Slurries, also known as watergels, Emulsions and ANFO mixtures require entrapped gas bubbles, tiny in size, for initiation and sustenance of the process of detonation. These bubbles or voids when compressed adiabatically by the shock-wave generated by the initiating charge behave as "hot-spots" and initiate the combustion reaction that finally leads to detonation. Incorporation of such voids can be done by several means that could be described under the following four broad categories:
--Incorporation into the explosive matrix materials containing entrapped voids, PA1 --physical means PA1 --mechanical action PA1 --chemical means
Under the first category, use of particulate materials such as hollow glass microballoons, plastic spheres, Perlites, volcanic ash, silicon sand, sodium silicate etc., is well known. This low density particulate matter retain the enclosed air even after mixing into the explosive and thus provide the hot-spots for detonation. Over the years it was found that while glass microballoons were effective performers others were not as effective. The glass microballoons on the other hand are expensive and pose handling problems due to their low bulk density.
In addition to the above, gas retaining agents such as foamed polystyrene, foamed polyurethane and the likes were disclosed in the U.S. Pat. No. 4,543,137. These were rigid or soft and spongy depending upon the resin employed for their preparation. However, the main problem with their usage is their participation in the combustion process as fuels leading to limitation in their usage levels or alternately undesirable oxygen-deficient situation.
The aspect of participation as fuel was taken care of in U.S. Pat. No. 5,409,556 by use of expanded grains such as expanded popcorn, expanded rice or expanded wheat for density reduction. It was claimed that these materials being carbohydrates were not good fuels and did not significantly alter the oxygen balance of the explosive composition when used in the small amounts required for density reduction.
Use of gas-in-liquid and gas-in-solid foams was disclosed in the Canadian Patent Applications 2093309 and 2113945 as means to incorporate voids. The foams consisted of mechanically or chemically generated gas locked in open or closed cellular structures. These foams contained, besides foam forming agents, substances such as TNT, Ammonium nitrate, Sodium nitrate etc., to further manipulate their sensitizing ability.
In the second category a new route to generating gas bubbles within the explosive matrix was disclosed in UK Patent Application 2179035. This involved dissolving the gas in the matrix under application of high pressure followed by sudden release to atmospheric pressure that then results in formation of gas bubbles due to the solubility differential for the gas in the matrix between high and ambient pressures. However, the main disadvantage of this method is that it requires specialized equipment for handling sensitized emulsions at high pressures.
In the third category we have the void generation by mechanical action. During prolonged mixing or intensive agitation voids get entrapped into the matrix due to its viscous nature. This method has the disadvantage that prolonged shearing could adversely affect long-term stability of the product.
The well-known alternative to above means of incorporating voids is to generate them in-situ by means of a chemical reaction. This technique, known in the industry circles as chemical gassing, is described in numerous patents viz., U.S. Pat. Nos. 3,886,010, 3,706,607, EP 0655430A1 to name a few. Several gas generating reactions involving chemical substances such as nitrites, weak acids, hydrazines and peroxides have been patented. One of the most commonly practiced reaction is that of sodium nitrite reacting with the ammonium nitrate present in the explosive matrix to produce nitrogen gas that gets entrapped in the form of bubbles in the viscous matrix.
The matrix of a water-in-oil emulsion explosive is prepared by mixing under agitation the aqueous and oil phases. The former phase contains salts such as ammonium nitrate, sodium nitrate, calcium nitrate etc., dissolved in water while the latter contains waxes, oils and emulsifiers. The emulsion matrix is prepared either by a batch process or a continuous process or combinations thereof employing different kinds of rotary mixers, static mixers, jet mixers, colloid mills, votators etc. The gassing agent is mixed into the emulsion matrix either in a rotary mixer or employing static mixers. Sometimes the aqueous solution of sodium nitrite is emulsified and incorporated as a water-in-oil emulsion to facilitate better mixing and derive benefits arising thereby as claimed in world patent WO 89/02881.
In the manufacture of watergel explosives, oxidizer salts such as ammonium nitrate, sodium nitrate, calcium nitrate etc are dissolved in water at above ambient temperatures and the resultant solution thickened (or gelled) using substances such as guar gum and subsequently crosslinked using metal ions such as chromium in suitable form. Optionally solid particulate materials like Aluminium are dispersed into the matrix. Here again gassing is usually the last operation that involves mixing in of aqueous solution of sodium nitrite.
Ammonium nitrate, in the form of porous prills as such can be mixed with fuel oil to form an explosive known as ANFO. While this itself is a popular explosive, in certain mining operations it is blended with water-in-oil emulsion in different proportions to render water-proofness as well as increased product density. The emulsion matrix used for blending is sometimes gassed to ensure satisfactory detonation performance of the final product. These products are generally known in the industry as Heavy ANFO.
Complementary to Heavy ANFO, we have what is known in the industry as doped emulsions. Here prills of ammonium nitrate or ANFO are mixed into the emulsion explosive matrix to enhance its performance. It is generally understood that in a doped emulsion the dopant constitutes a minor faction of the total composition.
The sodium nitrite--ammonium nitrate gassing reaction suffers from several drawbacks limiting its use. These are: difficulty in controlling the reaction rate, slowing down of the reaction at ambient and low temperature conditions, mass transfer problems causing the reaction to remain incomplete for several days etc.
The mass transfer problems arise due to the formation of discrete droplets when the aqueous solution of sodium nitrite is mixed into the emulsion matrix. While the mixing process is on there is a dynamic equilibrium between formation of new droplets and recombination of the existing droplets. During this process the sodium nitrite reacts rapidly with the ammonium nitrate it comes across but once the mixing process is stopped further reaction becomes diffusion dependent due to the two reactants being separated across the bilayer and slows down considerably. In the manufacture of cartridged explosives this situation could lead to continuation of the gassing reaction for several days after the cartridges are end-clipped leading to bursting of the cartridges.
Since the reaction is acid-catalysed, addition of acid can speed up the process of gassing but this method again has its limitations because highly acidic conditions lead to formation of undesirable oxides of nitrogen in place of nitrogen gas. The nitric oxide (NO) upon contact with air readily forms the dioxide (NO2) which being highly soluble in water results in disappearance of voids and density rise later.
Speeding up of the gassing reaction is also achieved by use, in conjunction, of additives such as thiourea and sodium or ammonium thiocyanate etc. But despite use of these additives that are known in the art as gassing accelerators, the gassing process slows down considerably when the process temperatures are near ambient. This aspect assumes much importance in today's mining industry in which explosives are more and more used in "Bulk" form.
In this scheme, the ungassed explosive matrix is transported to the mine bench where it is gassed and simultaneously loaded into the borehole saving much effort and material involved in sausage preparation. Thus it often becomes necessary to undertake the gassing at temperatures that could be well below the normal factory processing temperatures of 70-100.degree. C. and here slowing down of the gassing reaction with temperature becomes a major handicap. As a case of non-limiting example mining operations in a province like Himachal Pradesh are done with surface temperatures for most of the year remaining around 10.degree. C. Delayed gassing in such operations could cause hold-up of the subsequent operations such as stemming, priming of the holes etc.
Thus need is felt for a method of generating voids that is devoid of all the above mentioned problems and amenable to operation under cold climatic conditions. We describe here below such a method.